ELK CLONER, 1982: Regarded as the first virus to hit personal computers worldwide, "Elk Cloner" spread through Apple II floppy disks and displayed a poem written by its author, a ninth-grade student who was designing a practical joke.

BRAIN, 1986: "Brain" is the first virus to hit computers running a Microsoft Corp. operating system — DOS. Written by two Pakistani brothers, the virus left the phone number of their computer repair shop.

MORRIS, 1988: Written by a Cornell University graduate student whose father was then a top government computer-security expert, the virus infected an estimated 6,000 university and military computers connected over the Internet. Although viruses had spread over the Internet before, until "Morris" none was widespread.

MELISSA, 1999: "Melissa" was one of the first to spread over e-mail. When users opened an attachment, the virus sent copies of itself to the first 50 people in the user's address book, covering the globe within hours.

CODE RED, 2001: Exploiting a flaw in Microsoft software, "Code Red" was among the first "network worms" to spread rapidly because it required only a network connection, not a human opening an attachment. Although the flaw was known, many system operators had yet to install a software patch Microsoft made available a month earlier to fix it.

BLASTER, 2003: "Blaster" also took advantage of a known flaw in Microsoft software and, along with the 2003 "SoBig" outbreak, prompted Microsoft to offer cash rewards to people who help authorities capture and prosecute the virus writers.

SASSER, 2004: "Sasser" exploited a Microsoft flaw as well and prompted some computers to continually crash and reboot, apparently the result of bad programming. Although "Sasser" is hardly the last malicious software, the ones since then have generally received less attention as networks install better defenses and profit-minded virus writers try to avoid detection and removal of their works.